Environmental Engineering Reference
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the oxidation proceeds through outward migration of metal ions. In such cases,
fresh oxide is formed at the outer free surface in an unconstrained manner and
thus initial protective oxidation is expected under such conditions.
It has been pointed out by Borie et al. [53] that point imperfections in a grow-
ing oxide lattice may usually lead to a change in the lattice constants. It is to be
expected on the basis of elementary considerations of volume filling that the
average expansion would be positive for defects of the interstitial type but nega-
tive (i.e., contraction) for accommodating lattice vacancies as illustrated in Fig.
5.23. In the absence of other effects, the expansion or contraction would be iso-
tropic. However, this is a somewhat simplified view where each unit cell under-
goes expansion or contraction in an unconstrained manner. As additional oxide
forms, the defect density must gradually adjust throughout the already formed
oxide to a new steady-state value, and this adjustment will take place under severe
conditions of constraints. For example, a positive expansion in the directions
parallel to the parent metal interface may be prohibited because the oxide already
exists in a more or less uniform layer in these directions. This inhibition may
cause an expansion to occur primarily in a direction perpendicular to the parent-
metal interface. One approach that might prove fruitful is to compute the equiva-
lent local stress F(
ΞΎ
) that would be required to produce the average volume
Figure 5.23 Changes in the average unit cell size due to point defects. (The parameter
a 0 denotes the unstrained lattice parameter in one direction.) [Ref. 52].
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